2.2 Changing State - Evaporation

2.2 Changing State - Evaporation

Changing State - Evaporation

In the previous lesson, you saw that heat energy moves from a substance that is warmer to a substance that is colder through the process of 𝐜𝐨𝐧𝐝𝐮𝐜𝐭𝐢𝐨𝐧.

In conduction, the fast-moving particles in the warmer substance hit and transfer their energy to the slower-moving particles in the colder substance. This makes the particles in the colder substance speed up and the particles in the warmer substance slow down. The temperature of the colder substance increases and the temperature of the warmer substance decreases.

Changing State - Evaporation

In this lesson, you’ll see how heat makes molecules move fast enough to actually change from a liquid to a gas. This is the process of 𝐞𝐯𝐚𝐩𝐨𝐫𝐚𝐭𝐢𝐨𝐧 which you’ve seen many times in everyday life. We’ll explore evaporation all the way down at the molecular level.

Let’s take a look.

Multiple Choice

In the video, you saw a damp sidewalk become dry. If water is made of water molecules, what do you think happened to the molecules as the wet spot dried up?

• absorbed by the sidewalk

• evaporated into the air

Multiple Select

The process of water drying up like this is called evaporation. Check all examples of evaporation you may have noticed in your everyday life.

• My shirt got wet and it dried

• After it rained the water dried off the car

• Water level in a cup went down

• Hanging wet clothes out to dry

Multiple Select

You’ve probably noticed that heat speeds up how fast water evaporates. For example, a puddle seems to dry up faster on a hot day than a cold day. Check other examples you’ve noticed where heat speeds up the process of evaporation?

• Use a dryer to dry clothes

• Use a hair dryer

• Oven

Check Your Ideas

The wet surface dried as the water evaporated. When water evaporates, it changes from a liquid to a gas.

Notice that the word “evaporate” has the word “vapor” in it—water changes to water vapor but it is still water.

Common examples of evaporation are clothes drying in the sun or with the added heat from a clothes dryer, wet hair drying on its own or with the help of a hair dryer, or a puddle drying up in the sun.

Although you can’t see the water anymore after it has dried up or evaporated, it still exists. The water molecules separate and are in the air as a gas called water vapor.

Multiple Choice

For this experiment to be a good test, explain why it was important that the same amount of water was placed on each piece of brown paper

• It doesn't matter how much water was placed on the brown paper

• Equal amounts of water was needed because it was one of the variable that needed to be controlled

Multiple Choice

Think about the water molecules on the brown paper on both bags. Which water molecules were moving faster?

• the water molecules on the room temperature bag

• the water molecules on the hot bag

• neither, the water molecules were moving at the same speed on both bags

• neither, the water molecules are not moving

Multiple Choice

What do you think happened to the water molecules that were on the brown paper on the hot bag? Where do you think they went?

• Nothing happened to the water molecules on the brown paper on the hot bag they stayed put

• The water molecules moved slower on the hot bag than the cold bag and didn't move

• The water molecules moved very fast , spread out and then evaporated into the air.

Check Your Ideas

The type of paper towel material, amount of water, initial temperature of the water, and where the water is placed on the paper towel may all have an effect on the rate of evaporation. All these different factors are variables in the experiment. All these variables need to be kept the same so that the experiment is as fair as possible.

Even the surface each paper towel is placed on should be the same. This is why one paper towel is placed on a room-temperature bag instead of on a room-temperature table or desk. The only difference should be the amount of energy the paper towels are exposed to.

Check Your Ideas Continued

The water mark on the brown paper lying on the hot water bag disappeared faster than the mark on the paper lying on the room-temperature water bag. The water molecules on the hot bag were moving faster than those on the room-temperature bag.

Multiple Choice

In the animation, the water on the hot bag evaporates faster than the water on the room-temperature bag. Why do you think 𝐚𝐧𝐲 𝐰𝐚𝐭𝐞𝐫 𝐚𝐭 𝐚𝐥𝐥 evaporates from the paper on the 𝐫𝐨𝐨𝐦-𝐭𝐞𝐦𝐩𝐞𝐫𝐚𝐭𝐮𝐫𝐞 bag?

• There are still molecules moving just not as fast as the ones on the hot bag. So. the water will still evaporate just at a slower rate.

• There are no molecules moving just not as fast as the ones on the hot bag. So. the water will not evaporate just at a slower rate.

Multiple Choice

Look at the pictures on the previous slide and choose the best description of what is happening in each one on the molecular level.

• On a molecular level the hot bag has more energy that the room temperature bag and it is being transferred to the paper, which is then transferred to the water molecules. The water molecules are moving so fast that they evaporate

• On a molecular level the hot bag has less energy that the room temperature bag and it is being transferred to the paper, which is then transferred to the water molecules. The water molecules are moving so slow that they evaporate

Check Your Ideas

Adding energy increases the rate of evaporation. Heating water increases the rate of evaporation, and the drop of water that was heated evaporated first. Since the experiment controlled variables, heating water must increase the rate of evaporation.

Adding energy increases the motion of molecules. The water molecules on the paper towel on the warm bag are moving faster than the ones on the room-temperature bag. More of these faster-moving molecules break away from the other molecules and go into the air. The water molecules on the room-temperature bag will still evaporate, only slower because they have less energy.

Multiple Choice

How many hydrogen atoms are in one water molecule?

• 1

• 2

• 3

• 4

Multiple Choice

What differences did you notice between the ball-and-stick model and the space-filling model?

• The ball and stick model made it seem like the atoms were far apart and the space-filling model the atoms seemed to be closer together

• The ball and stick model made it seem like the atoms were close together and the space-filling model the atoms seemed to be far apart

Check Your Ideas

Water is made up of 1 oxygen atom (shown in red) and 2 hydrogen atoms (shown in gray).

The ball-and-stick model is used to highlight the angles at which the atoms are bonded together within a molecule. There is a "stick" connecting the atoms that are bonded together.

The space-filling model is used to highlight the space taken up by the electron cloud around the atoms within a molecule. The "balls" are attached directly, with no stick in between.

The shape of the water molecule and its attraction to other water molecules give water its characteristic properties.

Multiple Choice

True or False: Water molecules, as a liquid, are very spread apart because of their attractions for one another.

• True

• False

Multiple Choice

True or False: When water evaporated, the molecules broke apart into separate atoms.

• True

• False

Check Your Ideas

Water molecules, as a liquid, are very close together because of their attractions for one another but are able to slide past each other.

When water molecules attract each other, the oxygen part of one water molecule attracts the hydrogen part of another. The reason for this will be explored in detail later.

Now let's discuss water vapor. Water molecules, as a gas, are much further apart and usually just bounce off each other when they collide. When the water evaporated, the molecules themselves did not break apart into atoms. The molecules separated from other molecules but stayed intact as a molecule.

𝐊𝐞𝐲 𝐂𝐨𝐧𝐜𝐞𝐩𝐭𝐬 - 𝐂𝐡𝐚𝐧𝐠𝐞𝐬 𝐨𝐟 𝐒𝐭𝐚𝐭𝐞: 𝐄𝐯𝐚𝐩𝐨𝐫𝐚𝐭𝐢𝐨𝐧

1. Evaporation occurs when molecules in a liquid gain enough energy that they overcome attractions from other molecules and break away to become a gas.

2. Adding energy increases the rate of evaporation.

3. To conduct a valid experiment, variables need to be identified and controlled.

Multiple Select

Think of one of the demonstrations or experiments in this lesson that you found very memorable. Check all that apply that help to explain evaporation.

• Evaporation occurs when molecules in a liquid gain enough energy that they overcome attractions from other molecules and break away to become a gas.

• Adding energy increases the rate of evaporation.

• To conduct a valid experiment, variables need to be identified and controlled.

Poll

How do you feel about what you have learned in this lesson?

• 😊 Great! I am confident that I understand the material.

• 🙂 Good, but I still have a question. I will ask the teacher about it by email or in class.

• 😐 OK, but I need more practice. I will read through the lesson and re-watch some of the videos.

• 🤨 I need help. I will review the lesson and make a list of topics that are confusing to me. I will reach out to the teacher for help.